Offshore structures, such as those used for drilling and production of hydrocarbons as well as for generating renewable energy, in relatively shallows waters are typically supported by footings or foundations embedded in the seabed. Some of these footings are permanent whilst others are designed to be removable. For example, a modern rig for oil and gas drilling and well operations is usually mobile and its footings must then be removable. A typical mobile drilling rig for use in up to 120 m water depth has three supporting legs, with each leg being operable to be lowered or retracted independently through a jacking system located on its hull. The base of each leg comprises a shallow foundation or footing known as “spudcan”. The hull of the rig consists of ballast tanks, into which sea water is pumped to preload the rig and to allow the footings to penetrate into the seabed. Conventionally, the footings of some drilling rigs are equipped jetting nozzles, which are supplied with high pressure water from high pressure pumps; water jets from the nozzles loosen and fluidise the sandy seabed material during installation or removal of the footings. These conventional water jetting systems are notably applicable for liquefiable material such as sandy soil but has very limited effectiveness for cohesive soils such as clay.
FIG. 1A shows a known anchoring device by Nixon (see U.S. Pat. No. 4,086,866). The anchoring device is equipped with such conventional water jetting nozzles but has additional suction passageways. By supplying fluidizing water from the nozzles located near the lower part of the anchoring device and applying suction to the suction passageways, the seabed material immediately below the anchoring device is turned into a suspension, which is then pumped away through the suction passageways so that the anchoring device buries itself into the seabed.
FIG. 1B shows a known fluid actuated excavation system for installing sub-sea structures (see U.S. Pat. No. 5,259,458 issued to Schaefer). By providing a fluid stream from the top of a structure to the base, where a plurality of nozzles are terminated, jets of fluid ejecting from the nozzles excavate the installation area, thereby allowing the base of the structure to sit below the seabed. According to U.S. Pat. No. 4,086,866, this fluid excavation system is effective for seabed with sandy and granular material. In both U.S. patents, they disclose the use of fluidization of the seabed to install sub-sea structures but the effectiveness of the method for removing the embedded structures from the seabed is not discussed.
FIG. 2 shows another known fluid actuated excavation system for installing a footing into the seafloor and for its removal (see U.S. Pat. No. 4,761,096 issued to Lin). By providing internal jetting in a footing to shoot water out of nozzles, the surrounding seabed soil is fluidized and the seabed soil is thus loosened for the footing to penetrate into the seabed or for the footing to be pulled out. Preferably, the nozzles are directed at an angle so that the water jets are tangential to the footing's surface and this helps in removing soils away from the bottom of the footing. Despite being applicable for removing a footing that is embedded in the seabed, typically sandy seabed does not generate significant pulling resistance; this can be attributed to relatively shallow penetrations and high soil permeability; water jetting application thus is not instrumental for footing removal in such sandy soil conditions.
Despite development in the art of installing or retrieving a footing of a marine structure by fluidizing the seabed soil, there is a need for a new type of extraction system for relocating mobile marine structures, especially those structures that are installed in cohesive soil.